Journal of Sustainable Bioenergy Systems, 2013, 3, 18-32 Published Online March 2013 (
Microalgae Cultivation Using Offshore Membrane
Enclosures for Growing Algae (OMEGA)
Patrick Wiley1,2,3, Linden Harris1, Sigrid Reinsch4, Sasha Tozzi5,6, Tsegereda Embaye7,
Kit Clark7, Brandi McKuin1,2, Zbigniew Kolber5, Russel Adams8, Hiromi Kagawa7,
Tra-My Justine Richardson1, John Malinowsk i9, Co l in Bea l 10, Matthew A. Claxton11,
Emil Geiger9, Jon Rask10, J. Elliot Campbell2, Jonathan D. Trent4*
1Universities Space Research Association, Columbia, USA
2School of Engineering, University of California, Merced, USA
3PERC Water Corporation, Costa Mesa, USA
4NASA Ames Research Center, Moffett Field, USA
5Department of Ocean Sciences, University of California, Santa Cruz, USA
6Aurora Algae, Inc., Hayward, USA
7SETI Institute, Mountain View, USA
8Advanced Organic Methods, Penryn, USA
9Department of Mechanical Engineering, University of Nevada, Reno, USA
10Dynamac Corporation, NASA Ames Research Center, Moffett Field, USA
11Department of Chemistry, University of California, Santa Cruz, USA
Email: *
Received January 23, 2013; revised February 27, 2013; accepted March 19, 2013
OMEGA is a system for cultivating microalgae using wastewater contained in floating photobioreactors (PBRs) de-
ployed in marine environments and thereby eliminating competition with agriculture for water, fertilizer, and land. The
offshore placement in protected bays near coastal cities co-locates OMEGA with wastewater outfalls and sources of
CO2-rich flue gas on shore. To evaluate the feasibility of OMEGA, microalgae were grown on secondary-treated
wastewater supplemented with simulated flue gas (8.5% CO2 V/V) in a 110-liter prototype system tested using a sea-
water tank. The flow-through system consisted of tubular PBRs made of transparent linear low-density polyethylene, a
gas exchange and harvesting column (GEHC), two pumps, and an instrumentation and control (I&C) system. The PBRs
contained regularly spaced swirl vanes to create helical flow and mixing for the circulating culture. About 5% of the
culture volume was continuously diverted through the GEHC to manage dissolved oxygen concentrations, provide sup-
plemental CO2, harvest microalgae from a settling chamber, and add fresh wastewater to replenish nutrients. The I&C
system controlled CO2 injection and recorded dissolved oxygen levels, totalized CO2 flow, temperature, circulation
rates, photosynthetic active radiation (PAR), and the photosynthetic efficiency as determined by fast repetition rate
fluorometry. In two experimental trials, totaling 23 days in April and May 2012, microalgae productivity averaged 14.1 ±
1.3 grams of dry biomass per square meter of PBR surface area per day (n = 16), supplemental CO2 was converted to
biomass with >50% efficiency, and >90% of the ammonia-nitrogen was recovered from secondary effluent. If OMEGA
can be optimized for energy efficiency and scaled up economically, it has the potential to contribute significantly to
biofuels production and wastewater treatment.
Keywords: Biofuels; Wastewater Treatment; Microalgae; Photobioreactor; CO2 Mass Transfer; Fast Repetition Rate
Fluorometry; Instrumentation and Control
1. Introduction
Microalgae are currently under consideration as a sig-
nificant source of sustainable biofuels because of their
fast growth rate and ability to produce oil that can be
readily transformed into fuel [1,2]. These microscopic, sin-
gle-cell organisms can be cultivated on non-arable land,
lessening competition with agriculture and thus giving
them an advantage over other biofuel crops [3-5]. On the
other hand, microalgae require fertilizer and supplemen-
tal carbon dioxide (CO2) for optimal growth, which can
generate more environmental pollution and greenhouse
gas emissions than cultivation of more traditional biofuel
feedstocks, such as switchgrass, canola, and corn [6-8].
Several authors have noted that these environmental
*Corresponding author.
opyright © 2013 SciRes. JSBS
drawbacks can be ameliorated by linking microalgae
cultivation to wastewater treatment plants (to provide
water and nutrients) and flue gas sources (to provide
CO2), which also improves the economics and energy
return on investment (EROI) [6,9,10]. The feasibility of
constructing microalgae cultivation facilities close to
existing wastewater plants to avoid the prohibitive costs
of pumping water long distances will depend on the loca-
tion [11]. For most metropolitan areas, installing large
microalgae ponds or fields of photobioreactors (PBRs)
on land would significantly disrupt urban infrastructure.
For coastal cities, however, which use offshore wastewa-
ter outfalls, a system of floating photobioreactors (PBRs)
called Offshore Membrane Enclosures for Growing Al-
gae (OMEGA) may resolve this difficulty [12].
The proposed OMEGA system is designed to grow
freshwater microalgae in wastewater contained in flexi-
ble, clear, plastic PBRs attached to a floating infrastruc-
ture anchored offshore in protected bays [12-14]. The
offshore placement allows the system to be in close pro-
ximity to wastewater treatment plants and sources of flue
gas, eliminating the need to pump these wastes long dis-
tances to remote locations where land resources for algae
cultivation may be available. By using wastewater for
water and nutrients and by not using arable land the
OMEGA system avoids competing with agriculture or
disrupting urban infrastructure in the vicinity of waste-
water treatment plants. On a scale relevant to biofuels,
OMEGA will be intrusive in the marine environment,
although it is possible that a large flotilla of PBRs may
have beneficial effects in coastal areas. The OMEGA
system would remove nutrients from the wastewater that
is currently discharged into coastal waters and may there-
by mitigate “dead-zone” formation. The infrastructure
would provide substrate, refugia, and habitat for an ex-
tensive community of sessile and associated organisms
[15]. It is known that introduced surfaces in the marine
environment become colonized and can form “artificial
reefs” or act as “fish aggregating devices,” which increase
local species diversity and expand the food web [16,17].
A large-scale deployment of OMEGA systems may also
act as floating “turf scrubbers” and function to absorb
anthropogenic pollutants, improving coastal water qual-
ity [18].
The technical feasibility of the OMEGA concept how-
ever, has yet to be evaluated at any scale. Here a proto-
type, 110-liter OMEGA system was developed and tested
in a seawater tank, using freshwater microalgae and sec-
ondary-treated wastewater. The details of the system de-
sign are described, including the gas exchange and har-
vesting system as well as the essential monitoring and
control instrumentation. This OMEGA prototype main-
tained viable microalgae cultures, recovered ammonia-
nitrogen (NH3-N) from wastewater, and sustained areal
productivities at levels similar to those reported for other
cultivation systems. Furthermore, the prototype utilized
supplemental CO2 with greater efficiency than other cul-
tivation systems. These results support the proposal that
offshore microalgae cultivation, co-located with waste
resources, can contribute to the production of biofuels
without competing with agriculture [12,13].
2. Materials and Methods
2.1. Seawater Tank and Microalgal Cultures
Experiments were conducted in an 8800-liter seawater
tank at the California Department of Fish and Game,
Marine Wildlife Veterinary Care and Research Center in
Santa Cruz, CA (Lat: 36˚57'13'', Long: 122˚3'56''). The
tank was covered at night with a thermal pool blanket to
minimize heat loss. A mixed culture of green microalgae
used as the system inoculum was dominated by Des-
modesmus sp. and grown in 19-liter glass carboys con-
taining either BG11 medium (ATCC) or secondary waste-
water effluent. The carboys were aerated continuously
with a regenerative blower (Model VFC084P-5T, Fuji
Blowers, Saddle Brook, NJ) and periodically injected
with pure CO2 to lower the culture pH and provide a
source of carbon.
2.2. PBR System
Tubular PBRs contained swirl vanes to enhance mixing
by creating a spiral flow and were connected by pipes
and fittings to each other and to the rest of the circulation
system (Figure 1). The PBRs were constructed by weld-
ing sheets of 15-mil clear linear low-density polyethylene
(LLDPE) into tubes (I.D. 11.4 cm × 3 m long) using an
AIE double impulse foot heat sealer (Industry, CA). The
swirl vanes, improvised from polyethylene grain augers
(Lundell Plastics Corporation, Odebolt, IA) were fixed
inside a transparent schedule 40 polyvinyl chloride (PVC)
collar (O.D. 11.4 cm × 5.1 cm long) with a steel pin. The
sharp edges of the PVC collar were removed with a bench
grinder to prevent damaging the LLDPE. The swirl vanes
Figure 1. OMEGA photobioreactor (PBR) tubes with swirl
vanes. PBRs were made of flexible, clear LLDPE connected
with cam-lock fittings to a U-shaped PVC manifold. The six
swirl vanes (see insert enlargement) directed the flow into a
helical path to improve mixing and light exposure of the
Copyright © 2013 SciRes. JSBS
Copyright © 2013 SciRes. JSBS
were spaced 0.9 m apart and held in place using cable ties
wrapped around the collar on the outside of the PBRs.
The ends of the PBR tubes were attached to cam-lock
fittings (Model 400D, Banjo Corporation, Crawfordsville,
IN) and connected in series by a U-shaped manifold con-
structed of two schedule 40 PVC 90˚ elbows (10.2 cm).
The 10.2-cm cam-lock fittings on the PBR inlet and outlet
were reduced to 5.1 cm to accommodate the transparent
flexible PVC tubing that was connected to the suction and
discharge side of a centrifugal pump (Model 1MC1D5D0,
ITT-Goulds, Seneca Falls, NY). The speed of the cen-
trifugal pump was adjusted using a 1-HP GS-2 variable
frequency drive (Automation Direct, Cumming, GA). A
sensor manifold located before the pump inlet housed a
paddlewheel flow meter (Model 2537, Georg Fischer LLC,
Tustin, CA), pH probe (Model 2750, Georg Fischer LLC,
Tustin, CA), and dissolved oxygen (DO) sensor (Sen-
sorex, Garden Grove, CA) and provided connection to
the gas exchange and harvesting column (GEHC) (Fig-
ure 2).
2.3. Gas Exchange and Harvesting Column
The GEHC shown in Figure 3 was designed to: 1) man-
age concentrations of DO using an oxygen stripping de-
vice (OSD) based on a design by Barnhart [19]; 2) sup-
ply CO2 to the microalgae culture and control pH; and 3)
provide a settling chamber to collect aggregating micro-
algae for harvesting. Approximately 5% of the total sys-
tem volume was diverted to the GEHC per minute, using a
12 VDC SHUR-FLO diaphragm pump (Model 2088-343-
135, SHUR-FLO, Costa Mesa, CA). The pumping rate
into the GEHC was adjusted by changing the voltage set-
ting on the variable DC power supply (Model HY3005D,
Mastec Power Supply, San Jose, CA).
The culture from the PBR entered the GEHC through
the OSD section and cascaded over five stacked PVC
plates (20 cm2 each) housed in a pipe (schedule 40 PVC:
15.2 cm diameter × 0.3 m) attached to the top of the
GEHC with a rubber coupling (model 1056-63, Fernco
Inc., Davidson, MI). After the OSD, the culture entered
the gas-injection pipe (schedule 40 clear PVC 7.6-cm
diameter × 2.13 m), containing a CO2 diffuser made from
soaker hose (22 cm2) located 1.8 m from the top of the
column. The compressed CO2 source was a mixture of
8.5% CO2 in air (V/V) to simulate the concentration of
CO2 in typical flue gas [20]. The CO2 input was regu-
lated by a pH/temperature sensor (GF Signet 2750 pH
sensor electronics, Georg Fischer LLC, Tustin, CA).
After the gas-injection section, the culture enters the
settling chamber, which consisted of a section of clear
pipe (schedule 40 PVC 15.2 cm diameter × 0.91 m) with
a ball valve (1.3 cm) drain at the bottom. The culture
entered from the gas-injection pipe, which protruded 0.3
m into the settling chamber, and was capped to direct the
outflow to the sides and prevent resuspending biomass
collected at the bottom of the chamber. The culture re-
turned to the PBRs from the settling chamber through a
pipe (schedule 80 PVC 1.3 cm diameter) with a flow
meter (model F-40377LN-8, Blue-White Industries LTD,
Huntington Beach, CA) and a pneumatic pinch valve (1.3
cm VMP Series, AKO Armaturen & Separations GmbH,
Germany). The pinch valve maintained a constant liquid
height in the GEHC, using a feedback signal generated
by a pressure transducer (model PTD25-10-0015H, Au-
tomation Direct, Cumming, GA) in the settling cham-
2.4. Instrumentation and Control
A custom instrumentation and control (I&C) system was
constructed for process automation and data logging
(Figure 4). The pH and temperature sensors in the PBR
and GEHC were connected to a GF Signet model 8900
multi-parameter transmitter (Georg Fischer LLC, Tustin,
CA). Output signals from the transmitter, GEHC pressure
transducer, flow meter, and photosynthetically active
radiation (PAR) sensor were attached to inputs of a DL06
programmable logic controller (PLC) (Automation Direct,
Cumming, GA). The PLC transferred data to a human-
machine interface (HMI) created using LookoutDirect
software (Automation Direct, Cumming, GA) that dis-
played real-time data and allowed operators to specify
Figure 2. Inline sensors for pH, temperature, DO, and flow rate. The culture was pumped from the PBR past the sensors.
Part of the circulating flow was diverted to the GEHC (see Figure 3) at the GEHC suction fitting by a positive displacement
pump (not show n) and returned to the PBR flow at the GEHC return. The valved bypass was used to isolate the sensors for
leaning and maintenance without disrupting the overall circulation. c
Figure 3. Gas exchange and harvesting column (GEHC)
controls pH, removes settled microalgae and provides a
location for wastewater addition into the PBR system. An
oxygen stripping device (OSD, top) designed to remove ex-
cessive concentrations of photosynthetically generated dis-
solved oxygen was built into the GEHC. CO2 is added by
gas bubbles injected with the diffuser at a rate c ontrolled by
pH. Biomass collected in the settling chamber is removed,
whereas suspended microalgae are returned to the PBR
(return flow pipe, left). The pressure transducer controls a
pinch valve position to maintain a consistent liquid level in
the GEHC. The volume of the GEHC was periodically har-
vested from the drain valve at the bottom and replaced with
wastewater to replenish nutrients in the PBRs.
desired setpoints for the GEHC pH and liquid level.
Feedback control loops generated PLC output signals
based on the difference between the actual value and the
desired setpoint entered into the HMI. When the pH in
the GEHC exceeded the setpoint, the PLC output signal
adjusted CO2 injection rates through an Aalborg mass-
flow controller (MFC) (Aalborg, Orangeberg, NY). Si-
milarly, a current/pressure (I/P) transducer (Model IP610-
060-D, OMEGA Engineering Inc. Stamford, CT), regu-
lated by the PLC output signal, varied the pinch valve
position as needed to maintain the desired liquid level in
the GEHC. The objective of both control loops was to
minimize the difference between the actual value and the
desired set point. DO was measured using a Sensorex
DO probe (Sensorex, Garden Grove, CA) and data were
recorded using a Craig Ocean Systems (Ben Lomond,
CA) data logger. The physiological condition of the mi-
croalgae was monitored continuously using a fast repeti-
tion rate fluorometer (FRRF) set up for flow-through
2.5. CO2 Mass Transfer
The CO2 mass transfer efficiency for the GEHC was
Figure 4. Components of the I&C system. Inputs from the
sensors were routed through a multi-parameter transmitter
(A) or directly into a PLC (B) were transferred to a com-
puter database. Setpoint values established using an HMI
modulated PLC outputs that controlled a mass flow con-
troller for CO2 injection (C) and an I/P transducer (D) to
regulate pinch valve positioning.
calculated based on the column height and the gas flow
rate required to sustain a target microalgae productivity
of 20 g·m2·day1, in line with the average productivity
cited by Putt et al. [21]. Several authors have noted that
microalgae biomass is approximately 50% carbon [22-
24], a value corroborated by elemental analysis of the
algae grown in the OMEGA system (data not shown).
These values, together with a 2 × over design factor,
were used in Equation (1) to estimate a peak gas injec-
tion rate of 0.5 lpm into the GEHC.
Algae CarbonPBR
SolarCar 2
 
The CO2 mass transfer efficiency was quantified for
six different GEHC water column levels (0.3 m, 0.6 m,
0.9 m, 1.8 m, 2.1 m or 2.7 m) using a transparent PVC
test column (3 m × 7.6 cm). A diffuser (described above),
used to inject CO2 (8.5% in air, V/V) into solution, was
lowered to the bottom of the test column. The 0.5-lpm
gas injection rate (from Equation (1)) was controlled us-
ing a precision rotometer (Model WU-03218-52, Cole Pal-
mer, Vernon Hills, IL) calibrated with an Agilent ADM-
1000 Flowmeter (Agilent Technologies Inc., Wilmington,
DE). Tap water contained in a plastic barrel was weighed
using an Ohaus Defender scale (Ohaus Corporation, Pine
Brook, NJ) and the pH was adjusted to >11.00 with a
known mass of NaOH. The mass of water corresponding
to the desired liquid height was removed from the barrel
and added to the test column. The mass of CO2 dissolved
Copyright © 2013 SciRes. JSBS
into solution was determined by measuring the pH
change in the water column using the stoichiometry of
the acid-base reaction relationship between the NaOH
and described in Equations (2) and (3).
Na CO
HO C (2)
The CO2 uptake efficiency is the amount of CO2 ab-
sorbed in the GEHC column divided by the amount sup-
plied. The amount of CO2 absorbed was determined in-
directly by measuring pH changes in the water column.
The total moles of CO2 injected into the test column was
determined using Equation (4), which allowed the calcu-
lation of the mass transfer efficiency with Equation (5).
For this experiment, the mass transfer efficiency was cal-
culated based on the amount of CO2 required to change
the pH of the solution from 10 to 9, 9 to 8, 8 to 7 and
below 7.
CO (5)
A comparison of the CO2 mass transfer rate in the
GEHC and carbon consumption rate of microalgae in the
PBR gave a “detention time ratio” that estimates the
amount of time the culture can remain in the PBR before
carbon replenishment is needed. The overall mass trans-
fer coefficient (KLa) and subsequent CO2 mass transfer
rate in the GEHC were calculated from the titraion data
using Equations (6) and (7), whereas the carbon uptake
rate in the PBR was approximated with Equation (8).
Kaln CC
t t
 (6)
dc KaCC
dt (7)
Algae C
Uptak e
arbon PBR
Car Vol
Pf A
 (8)
Results from Equations (7) and (8) were used to cal-
culate the detention time ratio between the GEHC and
the PBR with Equation (9).
Xfer Rate
Uptak e
DTR (9)
2.6. System Inoculation, Sampling Protocol, and
Harvesting Procedures
Final plant effluent (FPE) was collected from the Santa
Cruz wastewater treatment facility mixed with inoculum
in a plastic barrel, and weighed with an Ohaus Defender
scale. The contents of the barrel were transferred into the
GEHC using a submersible pump. As the liquid level in
the GEHC approached the setpoint, the I&C system
opened the pinch valve and diverted liquid into the PBR.
The volume required to fill the entire system (~110L)
was determined by weight.
The optical density (OD750), NH3-N (Hach method
10031), NO3-N (Hach method 8039), and total suspended
solids (TSS) concentration (method 2540D) [25] were
measured on samples collected daily from a port located
on the discharge side of the PBR circulation pump. Dif-
ferences in the OD750 before and after physically shaking
the PBR to resuspend settled biomass were used to de-
termine the percent sedimentation within the PBR using
Equation (10).
After Before
750 750
SED 100
GrowthGEHC VolPBRVol Mass
The GEHC was drained into a barrel and refilled with
fresh FPE when the ammonia concentration approached
zero. The barrel was weighed to determine volume (as-
suming a density of 1 kg·l1) removed from the GEHC
and samples were collected for TSS analysis. The vol-
ume of water remaining in the PBR was determined by
subtracting the harvest volume from the total system
volume. This enabled calculation of the total biomass
produced between harvest periods, the biomass concen-
tration factor in the GEHC, and areal productivity (Equa-
tions (11)-(13)).
  (11)
Alga e
The result from Equation (11) and the totalized vol-
ume of gas injected into the GEHC recorded by the I&C
system were used to calculate the CO2-to-biomass con-
version efficiency with Equation (14).
Growth Car
Gas 2
CO 100
VpCO12g C
RT molCO
3. Results and Discussion
3.1. System Design and Performance
A 110-liter prototype OMEGA system was constructed
with two tubular PBRs floating in a seawater tank, con-
nected to an external GEHC and an instrumentation and
control system (Figure 5). The system components
Copyright © 2013 SciRes. JSBS
Copyright JSBS
(PBRs, GEHC, and I&C) are described in the Materials
and Methods. The PBRs, made of inexpensive plastic
(LLDPE), were tested for their ability to support photo-
synthesis. The GEHC served to control DO, provide CO2,
and remove and harvest microalgal biomass. The I&C
system monitored or controlled pH, temperature, flow
rate, and DO concentrations, recording sensor outputs
every three minutes.
Temperature and pH were measured both near the
outlet of the PBR in the sensor manifold (Figures 2 & 5)
and in the GEHC (Figure 3). The two monitoring sites
provided comparative data, and the GEHC pH sensor
served to control CO2 injection rates, using a setpoint of
pH 7.60. The I&C system also included measurements of
photosynthetically active radiation (PAR) and the effect
of light on cultures using FRRF, a rapid, nondestructive,
technique that detects variable chlorophyll fluorescence
in real time [26]. A decrease in the ratio of variable fluo-
rescence to maximum fluorescence (FV/FM) indicates a
decreased quantum yield resulting from damage to
photosystem II and is used as an index for photoin-
hibition [27]. Reported FV/FM ratios in cultures ex-
posed to high irradiance indicated up to 90% photoin-
hibition [27,28].
To limit sedimentation of microalgae in the PBRs,
cultures were circulated at velocities ranging from 14 to
21 cm·sec1, flow rates that reportedly prevent sedimen-
tation in open ponds [29]. Microalgae suspension and
mixing were enhanced by swirl vanes, which imparted a
helical flow pattern. With the combination of flow rates
and swirl vanes, microalgae settling in the PBRs never
exceeded 14% of the total biomass. The swirl vanes also
increased turbulence, which is known to improve nutrient
exchange rates and light exposure in PBR cultures [30].
In cultures grown in laminar flow systems photoinhibi-
tion and light limitations are observed, both of which
suppress productivity [28-30]. While swirl vanes may
have improved suspension and light availability and
hence productivity, two difficulties noted with the swirl
vanes tested were 1) increase biofouling on the walls of
the PBR in their vicinity and 2) increased drag, which
increased pumping energy.
To assess the performance of the prototype OMEGA
system, two consecutive experiments were conducted in
April and May 2012. Experiment 1 lasted 13.5 days and
experiment 2 lasted 8.6 days. In both experiments 1 and
2, the comparisons of hourly mean DO vs. PAR and DO
vs. Fv/Fm are shown in Figure 6. The increase in photo-
synthetically generated DO correlates well with PAR
from sunrise (06:00) to late afternoon (16:00), although
Figure 5. Component and flow diagram of the OMEGA system showing the circulation through the PBRs, sensor manifold,
and side loop for the GEHC.
© 2013 SciRes.
Figure 6. DO concentration, PAR and FV/FM values for Experiment 1 (left) and Experiment 2 (right). (Top) Mean hourly (±
SE) concentration photosynthetically generated DO (solid line) increases and decreases as a function of PAR (dotted line).
(Bottom) The mean hourly FV/FM ratio (dotted line) overlaying the range of data points (shaded area) measured by FRRF
indicates that the culture has maintained high photoconversion efficiency. The slight suppression of the FV/FM ratio during
mid-day is a result of photoinhibition caused by PAR intensity and elevated concentrations of DO (solid line).
the DO curve is artificially flattened at peak solar irradi-
ance (~12:00) because the DO values exceeded the up-
per threshold for the oxygen sensors (212% saturation)
(Figure 6, DO saturation). After 16:00, the decline in
DO was due to a combination of decreased photosyn-
thesis, respiration, and DO removal by the OSD in the
GEHC (see Materials and Methods: GEHC). The rela-
tive contribution of these different factors was not de-
At peak DO production and peak irradiance, there was
a slight photoinhibition indicated by FV/FM measure-
ments, which dipped to 0.49 in experiment 1 and 0.54 in
Experiment 2 (Figure 6, bottom). Rubio and co-workers
[31] noted that in long tubular PBRs DO buildup at high
irradiance caused photoinhibition and they identified this
as one of the greatest constraints on the scale-up of PBRs.
The solution for the OMEGA system is to adjust the ra-
tios of residence time in the PBR to the transfer fre-
quency to the GEHC, which depends on PBR length, the
number of GEHCs, and the flow rate. In the OMEGA
system the tested residence time of the culture in the
PBRs was 20 min, based on a PBR length of 3.1 m, a
4.5% transfer to the GEHC, and a PBR flow rate of 86 -
130 lpm. In the future, DO as it relates to photoinhibition
can be managed for PBRs of a given length using real-
time FRRF and DO data in the control logic algorithm to
modify GEHC input and flow rates. The size and con-
figuration of the OSD can also be modified to increase
the exchange of DO. In addition to DO management, the
GEHC was where CO2 was injected into the culture, both
as a source of inorganic carbon for microalgae growth
and to control the culture pH. Both carbon availability
and pH control are dependent on efficient CO2 delivery,
and both are critical to the productivity and economics of
large-scale microalgae cultivation [23,32-35]. Beal et al.
[36,37] have shown that commercial CO2 supply is one
of the biggest contributors to overall energy use and cost
of microalgal biofuel production.
Traditionally CO2 delivery systems, using sparging tubes
bubbling into shallow cultures, resulted in 80% - 90%
losses of CO2 to the atmosphere [21,38]. Diffusion me-
thods, using silicon membranes or hollow fibers reduce
CO2 loss to the atmosphere but are cost prohibitive and
prone to biofouling [21,33,39,40]. Bubble columns, like
the GEHC, are simple, low cost, and capable of reducing
CO2 losses to less than 20% [21,38].
3.2. GEHC Mass Transfer Efficiency and
Recycle Rate
The CO2 mass transfer efficiency in a gas exchange
column is influenced by the pH of the receiving liquid,
by the height of the liquid column, which determines
bubble contact time, by the size of the bubbles, which
determines contact area, and by the CO2 content of the
gas bubbles. Experiments with the GEHC indicated that
higher pH and a taller column increased CO2 mass trans-
fer efficiency (Figure 7). In the OMEGA system tested
here, however, site restrictions limited the gassing por-
tion of the GEHC to 1.8 meters, which gave a mass trans-
fer efficiency of approximately 50% for the operating pH
range in the GEHC of between pH 7.0 and 8.25. The
overall volumetric mass transfer coefficient (KLa)
Copyright © 2013 SciRes. JSBS
was 0.21 min1 (SE 0.01, n = 3), and the mass transfer
rate of CO2 was 1.69 × 104 mol·l-min1 (SE 1.03 × 105,
n = 3).
Assuming an areal productivity of 20 g·m2·day1, the
carbon consumption rate in the PBR was calculated to be
8.72 × 106 mol·l-min1. Balancing the mass transfer rate
in the GEHC with the carbon consumed by microalgae
would require one minute in the GEHC for every 20
minutes in the PBR. Therefore, 5 lpm (4.5% total system
volume perminute) were diverted from the PBR to the
GEHC for gas exchange. This pumping rate provided the
GEHC with an overdesign factor of 1.5 to ensure that
carbon consumption in the PBR did not exceed the injec-
tion capacity and limit microalgae growth.
3.3. GEHC Operation
Diverting only a portion of the culture for CO2 injection
resulted in a pH differential between the PBR and GEHC
(Figure 8, top). This differential was greatest at times of
the highest photosynthetic activity, which correlated with
the highest PAR and highest gas injection rate during the
day when most inorganic carbon was consumed (Figure
8, bottom). The control system could maintain the pH
near the setpoint (7.60), indicating that the mass transfer
rate of CO2 in the GEHC was not exceeded by the rate of
carbon removal in the PBR. Thus the control system
could monitor and deliver the amounts of CO2 demanded
by the microalgae. Furthermore, this system reduced CO2
losses as compared to “on-off” systems that produce hys-
teresis and potentially large variations from the de-
sired pH setpoint [22,32]. Further improvements in proc-
ess control may be realized using predictive models to
control pumping rates. Rubio et al. [31] developed a pre-
dictive model capable of estimating carbon depletion in
tubular bioreactors based on pH differential, which could
be adapted for the OMEGA system by comparing pH in
the PBRs versus the GEHC. Further research is needed to
determine how such pumping controls could improve
Figure 7. Efficiency of CO2 mass transfer in the GEHC
relative to the height of the column and the pH of the solu-
tion. Data were obtained (n = 76) experimentally using tap
water, pH adjusted (>11.0) with NaOH. For practical rea-
sons a maximum column height of 1.8 meters was used.
Figure 8. The mean hourly (± SE) pH, gas flow, and PAR recor de d during Expe ri ment 1 (le ft) and Experi ment 2 (r ight). Top:
pH values measured inside the GEHC (solid line) compared to pH in the PBR (dotted line). The differential between the
GEHC and PBRs increases during the day due to carbon assimilation for photosynthesis. The rate of CO2 injection was con-
trolled to maintain the GEHC pH setpoint during the day. The slow decre ase in pH at night is attributed to respiration. Bot-
tom: Gas flow rates (solid lines) indicating CO2 demand correlated with PAR (dotted lines), and inferred rates of photosyn-
hesis. The pH of the GEHC and PBRs equalize at night due to respiration. t
Copyright © 2013 SciRes. JSBS
energy efficiency and biomass productivity.
The details of harvesting intervals, biomass production,
and carbon utilization for both Experiments 1 and 2 are
given in Table 1. Harvesting occurred every 0.83 to 2.79
days, triggered by the depletion of NH3-N (see below). It
was noted that microalgae accumulated in the settling
chamber at the bottom of the GEHC hence the biomass
in the GEHC was higher than in the PBRs by a factor of
2.0 ± 0.1 (n = 7) in Experiment 1 and 1.4 ± 0.1 (n = 7) in
Experiment 2. These calculated concentration factors were
based on the total volume of the GEHC however, and
therefore do not represent the concentrations at the bot-
tom of the settling chamber.
Harvesting efficiency in the GEHC could be improved
by adding coagulants or by integrating an electrocoagu-
lation (EC) system, which produces coagulants in situ
[41,42]. The EC system is well suited for OMEGA be-
cause it has no moving parts and is easily automated [42,
43]. Furthermore, by adding a small amount of seawater
to the culture isolated in the GEHC, which would in-
crease its ionic strength, would lower the power required
for EC and electrolysis would produce electrolytic chlo-
rine, which could contribute to disinfecting the residual
water before release into the environment [43,44]. Addi-
tional research is needed to assess the EC harvesting
process for the OMEGA system.
3.4. Carbon Utilization and Biomass Production
The totalized volume of simulated flue gas (8.5% CO2/
91.5% air V/V) injected into the GEHC and the biomass
produced during Experiments 1 and 2 are shown in Fig-
ure 9. The changes in gas utilization, which appear as a
“staircase” in the plot, reflect the day/night cycles and
the on-demand input of CO2. The curve slopes upward
during light periods due to increased gas flow required to
satisfy the carbon demand for photosynthesis by the mi-
croalgae. The curve plateaus during dark periods when
there is no CO2 demand. The biomass produced relative
Table 1. Harvesting frequency, biomass yields and mass of carbon injected into the GEHC used to calculate carbon conver-
sion efficiency and areal biomass productivity during Exper iment 1 and 2.
Experiment 1
Elapsed Time,
Days between
Produced, g
Required, g Carbon Injected, gCarbon Conversion
Efficiency, %
1.85 1.85 5.2 2.6 5.8 45.0 4.0
2.83 0.98 8.4 4.2 8.2 51.3 12.3
3.66 0.83 2.6 1.3 4.1 31.6 4.5
4.79 1.13 13.4 6.7 13.1 51.1 17.0
6.73 1.94 23.1 11.5 18.0 64.2 17.1
8.75 2.02 15.3 7.7 12.9 59.4 10.9
9.68 0.93 11 5.5 10.1 54.5 17.0
12.5 2.79 29.3 14.7 19.6 74.9 15.1
13.5 1.06 15.2 7.6 14.5 52.5 20.6
Mean (SE) 13.7 (4.6) 6.9 (1.4) 11.8 (1.8) 53.8 (4.0) 13.2 (1.9)
Experiment 2
0.92 0.92 6.1 3.0 7.1 42.7 9.5
1.87 0.95 8.1 4.1 6.4 63.7 12.3
2.89 1.02 15.4 7.7 11.4 67.7 21.7
4.89 2.00 23.1 11.6 19.3 59.9 16.6
5.88 0.99 12.3 6.2 11.0 56.2 17.8
6.82 0.94 8.3 4.2 8.3 50.2 12.7
8.61 1.79 21.0 10.5 13.0 80.9 16.8
Mean (SE) 13.5 (2.5) 6.8 (1.3) 10.9 (1.7) 60.2 (4.7) 15.3 (1.6)
Copyright © 2013 SciRes. JSBS
Figure 9. Microalgal CO2 utilization and productivity in Experiment 1 (left) and Experiment 2 (right) with the day/night cy-
cle indicated by vertical stripes. Totalized gas flow (8.5% CO2 V/V) (bold black line) and biomass production (histogram).
The totalized gas flow has a “staircase” shape because CO2 was injected on demand; photosynthesis caused injection during
the day (slope up), but not at night (plateaus). The histogram shows biomass production in the height of bars (right axis, g)
and the time between harvesting in the width of the bars (bottom axis, days).
to the amount of CO2 injected was used to calculate the
CO2 utilization efficiency (Table 1): For Experiment 1
the mean efficiency was 53.8% ± 4.0% (n = 9) and for
Experiment 2 it was 60.2% ± 4.7% (n = 7), with values
from both experiments ranging from 31.6% to 80.9%.
These measured CO2 conversion efficiencies correspond
well to the CO2 solubility values obtained in the titration
experiment (see Section 3.3). Gas transfer in the OMEGA
GEHC could be improved by using a taller column
(greater contact time for rising bubbles), smaller bubbles
(greater surface-to-volume ratio), or higher CO2 concen-
trations. The site restricted column height, available equip-
ment determined the bubble size, and the CO2 concentra-
tion was chosen to simulate flue gas to determine if it
would be adequate to support microalgae cultures in the
prototype system.
The observed productivity, normalized to PBR surface
area per day, averaged 13.2 g ± 1.9 (n = 9), in Experi-
ment 1 and 15.3 g ± 1.6 (n = 7) in Experiment 2 (Table 1
& Figure 9 bars). In Experiment 1, sampling periods one
and three had low biomass yields. The initially low yield,
4.0 g·m2·day1, may have been due to a period of culture
acclimation. The second low yield on the third harvest
cycle (4.5 g·m2·day1) was due to a short incubation
period with minimal light exposure (Figure 9). Despite
these limitations, the average observed areal productiv-
ities were within the range of values reported for open
ponds [10,45,46], although somewhat less than those
reported for other PBR systems [5,47]. This disparity
with other PBRs may be due to lower nutrient concentra-
tions in the unsupplemented wastewater, the presence of
grazers and/or pathogens, or to other limiting culture
conditions (e.g., time of year or culture temperature).
Long-term experiments are required to determine the
limiting factors in the OMEGA system and its potential
3.5. OMEGA and Wastewater Treatment
The OMEGA system used secondary wastewater effluent
as a source of nutrients for microalgae cultures and the
concentrations of ammonia [NH3] and nitrate [3
were monitored (Figure 10). The rapid utilization of
NH3 required periodic replacement of spent culture me-
dium with fresh wastewater. Between 16% and 34% of
the total system volume was harvested from the GEHC
and replenished to increase the concentration of [NH3]
(Figure 10; top). While [NH3] followed a consistent pat-
tern of utilization and replenishment, the corresponding
] showed increases, decreases, or no change (Fig-
ure 10, middle). The increases in [3
O] were attributed
to nitrification by ammonia-oxidizing bacteria which are
known to be present in wastewater [48]. The decreases in
] observed in Experiment 1 (days 5 - 8) and Ex-
periment 2 (days 1 - 3 and 4 - 6) were attributed to the
depletion of NH3 and the utilization of NO3 as the micro-
algae’s secondary nitrogen source (Figure 10, middle).
Changes in preferred nitrogen sources have been ob-
served for other microalgae [49].
The calculated rates of ammonia removal varied, but
were positive, whereas the rates of nitrate removal were
both positive and negative; a “negative removal” rate
means nitrate production (Figure 10, bottom). The NH3
Copyright © 2013 SciRes. JSBS
Figure 10. Time course for the addition and utilization of
[] (top), [NO] (middle), and removal rates (bottom)
for Experiment 1 (left) and Expe riment 2 (right). As in Fig-
ure 9 the day/night cycle is represented by white/gray
shading and each line segment (top/middle) shows changes
in nutrient concentration from the time of wastewater addi-
tion to harvesting, corresponding to “biomass production”
bars in Figure 9. Removal rates (bottom) are shown as posi-
tive when nutrients were depleted or negative when nutri-
ent concentrations increased. The NH3 removal rates (black
bars) were always positi ve, but removal rates (grey
bar) were occasionally negative due to nitrification. The
microalgae preferred as their nitrogen source and
consume once the supply of was exhaust-
removal rate averaged 0.29 ± 0.04 (n = 12) and 0.49 ±
0.03 (n = 11) mg·11·hr1 for experiments 1 and 2, re-
spectively. In contrast, 3 removal rates were predo-
minantly positive during experiment 1 but predominantly
negative in Experiment 2. In both experiments the actual
nitrate concentrations represented the combination of pro-
duction and utilization at each sampling point. A more
effective utilization of total nitrogen may be achieved with
longer retention times.
These results indicate that microalgae growing in a
prototype OMEGA system can contribute to biological
nutrient removal in wastewater treatment. It is well es-
tablished that microalgae in ponds and other PBR de-
signs can effectively remove nutrients from wastewater
[50-53]. It has also been demonstrated that microalgae
can remove heavy metals [53,54] and organic contami-
nants, including surfactants, phenols, and hydrocarbons
[53,55-57]. Research reported elsewhere indicates that
the OMEGA system can also contribute to the removal
of pharmaceuticals and personal care products as well as
compounds of emerging concern [58].
Combining microalgae cultivation with wastewater treat-
ment can improve water quality and provide biomass for
biofuels or other products, but it remains to be demon-
strated that the economics and EROI of the combined
systems support its development [6,9,14].
4. Conclusions
OMEGA has the potential of co-locating microalgae cul-
tivation with two major waste-streams from coastal cities:
wastewater and CO2. By situating OMEGA systems in
the vicinity of offshore wastewater outfalls and CO2
sources, such as near-shore power plants, OMEGA can
transform these waste streams into resources that pro-
duce biofuels and treat wastewater without competing
with agriculture for water, fertilizer, or land [12]. The
experiments presented here explored the technical feasi-
bility of OMEGA, using a 110-liter prototype system that
was built and tested over a 23-day period. Microalgae in
secondary-treated wastewater circulated through PBRs
floating in seawater tanks and through a gas exchange
and harvesting column, while a custom I&C system mo-
nitored and controlled critical culture parameters. Ana-
lyses indicated that the system was supersaturated with
dissolved oxygen during the day due to photosynthesis,
but at the highest light levels there was only slight photoin-
hibition. The system rapidly used the 3
in waste-
water and had a CO2 conversion efficiency of >50%; bet-
ter than the 10% - 20% conversions in other systems
[21,38]. The areal productivity of the system averaged
14.1 g·m2·day1 overall with peaks above 20 g·m2·day1,
values consistent with reported US average microalgae
productivity of 13.2 g·m2·day1 [59]. The microalgae
consistently removed >90% of the 3
from the
secondary-treated municipal wastewater tested. This re-
sult, combined with observations that the OMEGA system
can remove other wastewater contaminants [58], suggests
that a scaled-up system could provide effective wastewater
treatment services.
Many open questions remain with regard to the feasi-
bility of large-scale OMEGA systems. The small-scale
prototype OMEGA system was intended for experimen-
tation and was not designed for energy efficiency or eco-
nomical scale up. For large-scale OMEGA deployment
dense configurations of PBRs, improved hydrodynamics,
optimized pumping and mixing, and more sophisticated
process control algorithms will be needed to increase
yields, improve EROI, and lower operating costs. In ad-
dition to the EROI and economics, questions about the
Copyright © 2013 SciRes. JSBS
impact of biofouling, concerns about engineering sys-
tems that can cope with marine environments, and envi-
ronmental issues around both environmental impact and
environmental regulations will need to be answered. It
remains to be seen if the need for sustainable biofuels
will drive the innovation necessary to address these ques-
tions to develop large-scale OMEGA systems.
5. Acknowledgements
The authors thank the OMEGA team members, in par-
ticular S. Ord, E. Austin, S. Fauth, A. Nazzal, A. Wong,
S. Harmsen, S. Eckhart, S. Martin, P. Buckwalter, J.
VanGelder, R. Tanakit, K. Acierto, Z. Hall, B. Smith, S.
Toy-Choutka, C. Young, S. Zimmerman, S. Marwood, K.
Long, S. Jayaprakash, M. Primack, W. Chen, J. Kaehms,
and C. Dye, as well as D. Jessup and M. Miller at the
California Department of Fish and Game, M. Ortega at
Santa Cruz Wastewater Treatment Facility, S. Lamerdin
and J. Douglas at Moss Landing Marine Laboratory, and
G. Engel and J. Powell at the San Francisco Southeast
Wastewater Water Treatment Plant. The OMEGA pro-
ject was supported by NASA-ARMD and the California
Energy Commission-PIER.
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QGas = Gas flow rate, lpm
PAlgae = Microalgal productivity, g·m2·day1
fCarbon = Fraction carbon in biomass
APBR = Area of the PBR tubes, m2
R = Ideal gas constant, 0.08206 L-atm mol-K1
T = Temperature, K
fAbs = Fraction CO2 absorbed
DSolar = Length of solar day, hours
MCar = Molar mass of carbon, g·mol1
pCO2 = CO2 partial pressure, atm
t = Time, minutes
= Moles of CO2
= CO2 mass transfer efficiency, %
MNaOH = Moles of NaOH
KLa = Overall volumetric mass transfer coefficient, min1
C* = Equilibrium [CO2], mol·l1
C = [CO2], mol·l1
PBRVol = Volume of PBR tubes, l
DTR = Detention time ratio, unitless
GEHCXfer Rate = GEHC CO2 mass transfer rate, mol·l1·
CUptake = Carbon uptake in the PBR, mol·l1·min1
AGrowth = g, Total biomass produced
TSSGEHC = Total suspended solids content of culture
harvested from GEHC, mg·l1
HVol = Volume of culture harvested from GEHC, l
TSSPBR = Total suspended solids content of the culture in
the PBR, mg·l1
IMass = Initial mass of solids in the system, g
HCF = Harvesting concentration factor, unitless
DHarvest = Harvesting frequency, days
CO2 to biomass conversion efficiency, %
VGas = Volume of gas injected into the GEHC between
harvest periods
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